Properties Of Gases And Diffusion Flashcards
Give an overview of the kinetic theory of gases
- gases are a collection of molecules moving randomly around a space
- Pressure is generated by collisions of molecules with a surface
- the more frequent and harder the collisions the higher the pressure generated by the gas
- Properties of gases can be described by the ideal gas equation, pV=nRT
What is Boyles law
Pressure (P) of a gas is inversely proportional to its volume (V) if temperature & number of gas molecules remains constant in a closed system
P is proportional to 1/V
Mechanism of inspiration & expiration is an example of Boyle’s Law
What is the partial pressure of a gas
In a mixture of gases, the total pressure = the sum of the partial pressures of the individual gases.
Partial pressure. Eg in a mix of 1/3 O2 and 2/3 N2
Partial pressure of O2 = 1/3 of Total pressure
Partial pressure of N2 = 2/3 of Total pressure
Total pressure = Partial pressure of O2 + Partial pressure of N2
What is atmospheric pressure
- pressure exerted by the weight of the air above the earth in the atmosphere
- At sea level: 101 kilopascals (kPa) = 1 atmosphere = 760 mmHg
- At high altitudes atmospheric pressure is lower (weight of air pressing down is less)
What is the composition of atmospheric air
Total Atmospheric Pressure at sea level = 101 kiloPascals
Partial pressure of O2 = 101 x 20.9% = 21.1 kPa
Partial pressure of N2 = 101 x 78 % = 78.7 kPa
Partial pressure of CO2 = 101 x 0.03%= 0.03 kPa
How do gasses diffuse
Gases dissolve and diffuse according to their partial pressure
• In the body gases diffuse down their partial pressure gradient
• from area of high partial pressure – to low partial pressure e.g. movement of oxygen from alveolar air ↔ blood
• Partial pressures (rather than concentrations) used to describe gases in the body
• Denoted by ‘p’ - as in pO2
What happens when inspired gases come in intact with body fluid
- Gas molecules will enter water to dissolve in liquid
* Water molecules evaporate to enter air
What happens at equilibrium in terms of vapour pressure of water
- Water molecules entering the air exert ‘vapour pressure’. At equilibrium,
- rate of H2O evaporation = rate of H2O condensation
- the air is saturated with vapour
- Saturated Vapour Pressure (SVP) = 6.28kPa at body temp.
- Inhaled air becomes saturated with water, in the upper respiratory tract
Describe the partial pressure of gases in moose air
How does the water vapour affect the partial pressure of the other gases?
• Saturated vapour pressure = 6.28 kPa at body temperature
• Pressure of the rest of the gases = 101 – 6.28 = 94.7 kiloPascals
• They are still in the same ratios as in dry air
So pO2 = (101 - 6.28) x 20.9% = 19.8kPa
pN2= (101 - 6.28) x 78 % = 73.8 kPa
pCO2=(101 - 6.28) x 0.03%= 0.03kPa
What happens at equilibrium in terms of gases dissolving in body fluids
Gas dissolves in body fluids
• Dissolved gas molecules also exert pressure in the liquid
• Equilibrium is reached when: rate of gas entering water = rate of gas leaving the water.
At equilibrium,
• the partial pressure of the gas in the liquid = partial pressure of the gas in the air above it
Give a summary of gases in liquid
• Partial pressure is the pressure exerted by the dissolved gas in the
liquid At equilibrium, • the partial pressure of the dissolved gas in the liquid = the partial
pressure of the gas it is exposed to
• Note: another term used for partial pressure of a gas in the liquid is
‘tension’ (e.g. oxygen tension in blood)
How is partial pressure different to the amount of a dissolved gas
Amount dissolved = partial pressure x solubility coefficient of gas
(Solubility coefficient – is a constant for the individual gas and the solvent)
Solubility coefficient of O2 in plasma = 0.01 mmol/ L /kPa (at 37°C) o
So When exposed to a pO of 13.3 kPa (as in alveolar air)
- 0.01 x 13.3 = 0.13 mmol of O2 will dissolve
Plasma has 0.13 mmol dissolved oxygen /per litre. 0.01 x 13.3 = 0.13
What happens if a gas reacts to a component of the liquid in Addison to dissolving and give an example
If a gas reacts (e.g. O component of the liquid in addition to dissolving, this reaction must complete before equilibrium is reached and partial pressure is established.
• O2 enters plasma & dissolves in it
• dissolved O2 enters RBC to bind to Hb
• Process continues till Hb fully saturated
(each Hb molecules binds 4 O2 molecules)
• after Hb is fully saturated, O2
dissolve till equilibrium is reached
• At equilibrium, pO2 of plasma = alveolar air
How is oxygen bound to Hb downloaded into tissues
• Blood contains both dissolved and Hb bound
oxygen
• The pO is a measure of dissolved O2 in the blood.
• Dissolved O2 is available to diffuse into tissues down its partial pressure gradient
• As dissolved O2 leaves the blood, it will be replaced
By O2 bound to Hb.
• In this way, the oxygen bound to Hb will bedownloaded and diffuse into tissues
Define partial pressure
Partial pressure = pressure exerted by the dissolved gas in a liquid
Define conc of a dissolved gas
Concentration of dissolved gas = number of mmol of gas, dissolved in a litre of liquid
Define solubility coefficient
Solubility coefficient = a constant for each gas (solute) and solvent
What is the total content of gas
Total content of gas = dissolved gas + gas bound to or reacted with a component
How do the partial pressures of o2 and co2 in alveolar air comaore to in inhaled air
In alveolar air :
pO2=13.3 kPa (lower than inhaled air)
PCO2=5.3 kPa (higher than in inhaled air)
Because
• Inhaled air mixes with residual volume
• Effect of O2 diffusing across the alveolar wall
• effect of CO2 entering the alveoli
Alveolar air composition stays constant around this level; Blood equilibrates to this level
Describe the gradient s in moved venous blood compared to alveolar air
• Alveolar PO2 > PO2 in mixed venous blood
• Alveolar PCO2 > PO2
in mixed venous blood
• so oxygen will diffuse into blood and carbon dioxide out
What affects the rate of diffusion
• partial pressure difference (gradient) across membrane (P1-P2)
• A - the surface area available for diffusion
• T – (thickness) i.e. distance. molecules must diffuse
• Diffusion coefficient of the individual gas:
• The solubility of the gas in the liquid : greater the solubility, faster the rate of diffusion
• Molecular weight of gas:
– Higher the molecular weight slower the rate of diffusion
Diffusion coefficient (D) is proportional to solubility/ root molecular wt
• used to determine the relative rates at which different gases will diffuse across the same membrane at the same pressures;
Compare the diffusion of CO2 vs O2
- Solubility: CO2 much more soluble than O2 (so diffuses faster)
- Molecular weight: molecular weight of CO2 > O2 (slows down CO2)
Combine the two factors Oxygen is small and thus fast, but CO2 is more soluble Overall, the effect of solubility is greater
CO 2 diffuses 20 times faster than O2
than CO
• Larger difference in partial pressures (ΔP) compensates for slower diffusion of O2
• In a diseased lung, O2 gas exchange is thus more impaired than CO2, O2 slower diffusion rate
What are thelayers of diffusion barrier from air to rbc
- Fluid film lining alveolus
- epithelial cell of alveolus
- Interstitial space
- endothelial cell of capillary
- plasma
- red cell membrane
- 5 cell membranes
- 3 layers of cytoplasm
- 2 layers of tissue fluid +plasma
Describe the surface o the alveolar capillary
• The surface area of the alveolar capillary membrane about 100 m2 • Barrier < 0.4 μM thick • oxygen exchange complete in 1/3 of time blood spends capillary • so plenty of reserve – for exercise
What factors affect rate of gas diffusion in disease
• Thickness of the membrane
– Increase as a result of oedema fluid in the interstitial space and in alveoli
– Lung fibrosis - increased thickness of alveolar capillary membrane
• Surface area of the membrane
– decreased by removal of an entire lung
– Emphysema - decreased surface area
• Diffusion coefficient of the gas:
– CO2 always diffuses much faster than O2
– So, diffusion of O2 affected → pO2 is low
– Diffusion of CO2 not affected → pCO2 is normal
What are probelms with alveolar capillary membrane
See slide
